The Mystery of His Will: Summing Up All Things in Christ

I am presently reading The Pillar New Testament Commentary on THE LETTER TO THE EPHESIANS by Peter T. O'Brien. Surprisingly, I had not considered that the mystery mentioned by Paul in Ephesians (and other letters) is God's plan to 'sum up all things in Christ.' This summing up of everything is indeed a mystery, hard to fathom.
O'Brien writes:

His 'mystery', though not understood by men and women, had been planned by him, the Creator, from eternity.

I couldn't agree more. God, in infinite wisdom, planned everything. Nothing is outside his foreknowledge.
O'Brien continues:

Before the foundation of the world he chose a people for himself in Christ and predestined them to be his children (Eph 1:4-5)...God has not changed; nor is he abandoning his first creation (i.e the Jews) by forming a new creation in Christ (i.e. Gentile Christians together with Jewish Christians). Salvation and the unity of the Jew and Gentile in Christ have always been his purpose...And he who created all things in the beginning with this goal in mind will consummate his work of re-creation on the final day when he brings all things together in unity in his Son, the Lord Jesus (1:10).

All things? I struggle sometimes with what Paul means when he writes 'all things'. At the end, will everything become as it was at the creation? Except that everything will know and be of Christ?
The mystery of his will is the summing up of all things in Christ.